Internet Control Message Protocol
Internet Control Message Protocol (ICMP) defined by RFC 792 and RFC 1122 is used for network error reporting and generating messages that require attention. The errors reported by ICMP are generally related to datagram processing. ICMP only reports errors involving fragment 0 of any fragmented datagrams. The IP, UDP or TCP layer will usually take action based on ICMP messages. ICMP generally belongs to the IP layer of TCP/IP but relies on IP for support at the network layer. ICMP messages are encapsulated inside IP datagrams.
ICMP will report the following network information:
- Network congestion
- Network errors such as an unreachable host or network.
The ping command is also supported by ICMP, and this can be used to debug network problems.
The ICMP message consists of an 8 bit type, an 8 bit code, an 8 bit checksum, and contents which vary depending on code and type. The below table is a list of ICMP messages showing the type and code of the messages and their meanings.